ajax in wordpress plugin without "wp-admin/admin-ajax.php"

I want in wordpress 4.2 to make plugin with ajax requests and have some counfusions: I googled and found some docs like http://ift.tt/1EMcdfe

this file for ajax functions called is "wp-admin/admin-ajax.php", but the thing is that I did not the idea of writing my changes in common wp file.

Which is the better and right way ? please, give ref to examples ...

Thanks~